Messerschmitt Bf.109 Prototypes - Participation in the Spanish Civil War

The Messerschmitt Bf.109 began its career fighting since the very beginning. In fact, the prototypes, from the V3 onwards, served with the Condor Legion, the German volunteers serving with the Aviación Nacional (Nationalists' Air Force).
That way, the V3, which was quite distinctive thanks to its extra cannon firing through the propeller. It served in the Spanish Civil War with the spinner modified to the V4 standard, as it was better. The V4 and V5 resembled the Bf.109A and they were modified to that standard even if it wasn't an homogeneous version. All prototypes served with the VJ/88 that was the test squadron of the Legion.
The A version seved with the fighter unit of the Condor Legion, the J/88 and many times they have been misidentified with the Bf.109B version. They were marked from 6-1 to 6-16 and served in the early part of the conflict, specially in the northern front. All of them were either modified to the B standard or withdrawn from service. One of them marked as 6-15, ran out of fuel and had to land on Guadalajara, North of Madrid, that was still part of the Republic. It was sent to Sabadell, in Catalonia, to be tested by Republican and French pilots and sent by ship to the USSR where it was furtherly tested and its fate is unknown.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_variants
2. http://bioold.science.ku.dk/drnash/model/spain/bf109.html
3. Salamander Books - The Complete Book of Fighters

Messerschmitt Bf.109 - Prototypes

The design work on the Bf.109 began in March 1934, three weeks after the main development contract was awarded. By January 1935 two mock-ups were ready and the RLM (Third Reich's Air Ministry) designated the design as the Bf.109, the next number they had available for the BFW.
The first prototype, V1, was completed in May 1935 but, due to shortage of German engines, the RLM adquired four Rolls-Royce Kestrel VI engines in exchange of a Heinkel He.70 to be used as an engine test-bed. Messerschmitt fitted the Kestrel VI engine into the Bf.109V1 airframe and it flew for the first time in late May 1935 at the airfield located south of Augsburg, south of Bavaria, piloted by Hans-Dietrich "Bubi" Knoetzsch. After four intensive test months, the aircraft was delivered to Luftwaffe's test centre in Rechlin.
In 1935 the first Junkers Jumo engines were available so the second prototype, designated V2 was completed with that engine in October 1935. The Junkers Jumo 210A engine was rated at 600hp and it also powered the third prototype, V3 that, due to a delay in the next batch of Junkers Jumo 210A engines, didn't fly until May 1936 and it was the first one to be armed. Initially it was armed with just two cowl-mounted MG-17 machine guns, but as the RLM got news that the Royal Air Force had started to develop and manufacture the Supermarine Spitfire, that was armed with eight machine guns, they decided to fit a third machine gun firing through the propeller hub.
The Bf.109A was the first version with very few machines manufactured, in fact its existance has been questioned many times, but apparently it did exist. It was powered by a Junkers Jumo 210B  engine rated at 631hp (471Kw) driving a two-bladed propeller and armed with two cowl-mounted 7.92mm (0.312in) MG-17 machine guns. Soon, some of them were switched to the Jumo 210D rated at 661hp (493Kw) engine that was better. Given the many changes the Bf.109A went through, it has been regarded as a non-uniform type and that's why its existance has been questioned. Visible changes included engine, cockpit, machine-gun ventilation holes and slats, and the location of the oil cooler was changed many times to prevent overheating.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_variants#Bf_109_A.2FB.2FC.2FD
2. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109